I have an rain bird CP-100 one inch that I intend to use on a tennis ball cannon. The solenoid is mounted off center over the diaphram. Couldn't I just remove the solenoid, drill out the hole under the solenoid and then JB weld a pipe fitting into this solenoid hole to attach a manual dump valve?

There should be a small manual bleed screw on the center of the valve. Un scerw it then drill a 7/16" hole if you are using a tap or a 1/2" hole if you aren't. Then screw in the nipple and blowgun. Then remove the solinoid and fill the hole with epoxy.

plug the center hole of the solenoid port with some glue. widen the side hole as much as possible. Epoxy a 3/4" threaded screw pulg thing in the solenoid hole. Tap a 1/4" male to male adapter into that. Attatch that to a 1/4" street elbow. then stick on a blowgun. Epoxy the small manual bleed screw back in and cut off the top.
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